1. Field of the Invention
This invention relates to an optical instrument having the zooming function.
2. Related Background Art
In recent years, there has been proposed a function of keeping the size of an object which is occupied in the picture plane (hereinafter referred to as the auto zooming function) always constant even if the object distance changes.
Heretofore, such auto zooming function has been provided by calculating a focal length f to be assumed from the object distance R and the size x of the object which is occupied in the picture plane, and setting a lens to the calculated focal length f.
Also, as a simple method, in Japanese Patent Application No. 1-2836 by the applicant, a distance ring and a zoom ring are disposed at a predetermined initial position, whereafter AF is started from that position and the pulse number of a rotatable pulse board operatively associated with an AF motor or the like is counted, whereby the absolute position of the distance is known and also, the pulse number of a rotatable pulse board operatively associated with a zoom motor or the like is made coincident with the pulse number of the AF motor, whereby auto zooming is established. That is, in the case of a four-group zoom lens of the most popular forward lens focusing type, the relation between the amount of axial movement S of the forward lens and the object distance R is substantially such as shown in FIG. 2 of the accompanying drawings. Also, the relation between the angle of rotation .theta. of the zoom ring and the focal length f is shown in FIG. 3 of the accompanying drawings. In this case, when the ranges A and B in the figures are suitably selected, the values of f.sub.2 /R.sub.2 and f.sub.1 /R.sub.1 become equal to each other and it becomes possible to make .theta.=.theta..sub.1 +A.times.(.theta..sub.2 -.theta..sub.1) coincident with f/.theta. when S=S.sub.1 +A.times.(S.sub.1 -S.sub.2) (0.ltoreq.A.ltoreq.1). Accordingly, if the patterns and gear ratios of the two pulse boards are set so that the pulse number of the AF motor generated in the area A and the pulse number of the zoom motor generated in the area B may be the same number, and .theta..sub.1 and S.sub.1, or .theta..sub.2 and S.sub.2 are set to the initial reset positions and further, the pulse count values here are both rendered into O, then the auto zooming function will be established by thereafter making the pulse number of the zoom motor coincident with the pulse number of the AF motor (however, when the design is made such that the pulse number of the zoom motor increases for wide .fwdarw. tele, the AF motor is designed such that the pulse thereof is increased by the rotation in the direction of close distance.fwdarw.infinity).
In the above-described system, however, the driving, stoppage and reversal of the AF motor are all replaced by those of the zoom motor and therefore, magnification change becomes stiff, and this has also led to a case where the picture plane becomes very unsightly.